I have attached an excel file of historical classifications by income from World Bank. But it contains only for fiscal year from 1989 to 2018.
According to WB, the low-, middle-, and high-income group thresholds were established in 1989 based largely on operational thresholds that had previously been established. "... the staff report 'Per Capita Income: Estimating Internationally Comparable Numbers,' presented to the Executive Directors in 1989, established an explicit benchmark of $6,000 per capita in 1987 prices."
I guess that's why they don't have data before 1989.
Hope that still helps!
(sorry the pc does not allow me to type in Chinese :(((
Relevant link:
https://datahelpdesk.worldbank.org/knowledgebase/articles/378833-how-are-the-income-group-thresholds-determined